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Biogas generation: developments. problems, and tasks - an overview

E.J. DaSilva

Division of Scientific Research and Higher Education, Unesco, Paris, France

Introduction

In recent years, biogas systems have attracted considerable attention as a promising approach to decentralized rural development. Developed and developing countries and several international organizations have shown interest in biogas systems with respect to various objectives: a renewable source of energy, biofertilizer, waste recycling, rural development, public health and hygiene, pollution control, environmental management, appropriate technology, and technical cooperation. Within the context of the UNEP/Unesco/ICRO microbiology programme, which is sponsored jointly by the United Nations Environment Programme, Unesco, and the International Cell Research Organization, several workshops have already been held in Yogyakarta, Manila, Mexico City, Singapore, and Bangkok, in an attempt to catalyze the applications of this acknowledged low-cost, nonwaste-producing technology that is increasingly being deployed to manage the environment and to ameliorate the search for substitute sources of fuel, food, and fertilizer (1 - 4). Early in 1979, in joint co" operation with IFIAS and ESCAP/UNIDO, a workshop will be held at Bandung to deal specifically with village micro" biology and the integrated biogas farming system. In this context, it is hoped that this activity on "The State of the Art of Bioconversion of Organic Residues for Rural Communities," a UN University joint World Hunger-Natural Resources activity receiving Unesco and UNEP/Unesco/ ICRO Panel support, will be making a significant contribution to the application of bioconversion processes for rural communities.

The utilization of microbial activity to treat agricultural, industrial, and domestic wastes has been common practice for a half century. Treatment includes the aerobic, activated sludge process and the anaerobic or methane fermentation method; the latter is simple, does not require imported know-how or components, is suited to small family or village-scale digestion, and is the only process utilizing waste as a valuable resource. Of great importance to the developing countries, the use of methane has, until recently, been restricted because of public antipathy or because other, cheaper energy sources were available. But, as can be seen from Tables 1 and 2, biogas technology today is a sufficiently significant producer of energy to command the attention of a fair number of countries (5) and agencies.

TABLE 1. Promotion of Research and Development Related to Use of Biogas

Sources: S.K. Subramanian, Bio-Gas Systems in Asia, Management Development Institute, New Delhi, 1977; U.N. Agency documents.

TABLE 2. International Agencies Engaged in Biogas Research, Training, and Development Programmes

Sources: S.K. Subramanian, Bio-Gas Systems in Asia, Management Development Institute, New Delhi, 1977; U.N. Agency documents.

What is biogas?

Methane is the main constituent of what is popularly known as biogas. A colourless, odourless, inflammable gas, it has been referred to as sewerage gas, klar gas, marsh gas, refuse-derived fuel (RDF), sludge gas, will-o'-the-wisp of marsh lands, fool's fire, gobar gas (cow dung gas), bioenergy, and "fuel of the future." The gas mixture produced is composed roughly of 65 per cent CH4, 30 per cent CO2, and 1 per cent H2S. A thousand cubic feet of processed biogas is equivalent to 600 cubic feet of natural gas, 6.4 gallons of butane, 5.2 gallons of gasoline, or 4.6 gallons of diesel oil. For cooking and lighting, a family of four would consume 150 cubic feet of biogas per day, an amount that is easily generated from the family's night soil and the dung of three cows. In addition, rural housewives using the biofuel are spared the irritating smoke resulting from the combustion of firewood, cattle dung cakes, and the detritus of raw vegetables (Figure 1).

Figure. 1. The Attributes of Biogas as a Fuel vs. the Disadvantages of Wood (Source: Bio-Gas Newsletter, August 1976)

Microbiology of CH4, or bio-methanogenesis

Anaerobic digestion technology or the methane-generating bioconversion yields both fuel (biogas) and organic fertilizer (sludge), products that are the final result of microbial action on cellulosic and other non-chemically processed organic residues. These substrates are obtained through a series of degradative steps that involve a variety of bacteria (6 11). In the first step, complex polymeric organic substrates - proteins, carbohydrates, and fats - are transformed by non-methanogenic bacteria into essentially non-methanogenic substrates like butyrate, propionate, lactate, and alcohol. Through a second step that involves the acetogenic bacteria, the composition and identity of which still remain to be determined, these compounds are transformed into methanogenic substrates, i.e., acetate, H2 and C1 compounds that are converted into CH4 and CO2 by the methane bacteria, obligate anaerobes that multiply in a neutral or slightly alkaline environment.

That the smooth cooperation of the three groups of bacteria has to be well regulated is exemplified by Bryant's discovery (12) of two mutually inter-dependent species existing in a symbiotic association that was formerly considered a pure culture under the name of Methanobacillus omelianskii. The association is comprised of two symbionts: an acetogenic organism and a methanogenic organism. The acetogen produces acetate and H2 and CO2, thereby disrupting the process of auto-inhibition with the acetogen, which succumbs to the H2 it produces.

Again, it is necessary that both aspects of the anaerobic digestion process - liquefaction and gasification - be well balanced. If the methane bacteria are absent, the digestion process may only succeed in liquefying the material and may render it more offensive than the original material. On the other hand, if liquefaction occurs at a faster rate than gasification, the resultant accumulation of acids may inhibit the methane bacteria and the bioconversion process as well.

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